It has long been recognized in the petroleum industry that addition of heat to the productive interval in oil wells can be very beneficial to stimulating and maintaining the production rates of high viscosity heavy oil and waxy oil.
Steam injection is used extensively, but has certain inherent characteristics that makes it disadvantageous to use under certain circumstances. For example, some oil bearing reservoirs also contain clay minerals which swell in contact with fresh water. This swelling damages the permeability of the reservoir rock and, therefore, its fluid productivity. In many oil producing regions, fresh water supplies for generating steam are limited. The condensed water from the injected steam that is produced with the reservoir fluids must be separated and extensively treated to reuse it for steam generation or to dispose of it to near-surface aquifers. In oil reservoirs that are more than a few meters thick, injected steam enters the reservoir at its most permeable point, thus heating the region near that point, but leaving large sections of exposed productive reservoir unheated.
An electrical heating system for well conditioning does not need water injection thereby eliminating clay swelling permeability problems, water supply, treating, and disposal as considerations and the addition of heat may be beneficial in reducing existing clay swelling. On the other hand the system may use water, convert water to steam or use other fluid, if advantageous to increase production, to destroy contaminants, to promote fracturing or otherwise condition the well. The invention may be of any required length and be configured to have variable or constant heat release along the length thereby enabling heating of the entire productive zone, and beyond, at variable total as well as variable incremental heat rates consistent with requirements.
Several configurations of electrical apparatus have been proposed and tested in the field to thermally stimulate oil producing reservoirs. One of the first methods implemented was the suspension of electrical resistance heating elements on an electrical power cable across from the interval to be heated. Electrical current is delivered through the cables to the resistance elements causing the resistance elements to increase in temperature in proportion to their electrical resistance and the square of the electrical current passing through them. Heat is transferred to the produced fluid by convection from the surface of the resistance elements, thereby raising the temperature of the fluid in the well annulus. This increase in temperature causes some heat to be transferred by conduction through the wall of the well's production casing, or liner, to the near wellbore region of the reservoir. The temperature rise in the near wellbore region causes a reduction in the viscosity of the oil flowing in that region, with a consequent reduction in pressure drop there and an increase in productivity due to the reduction in flow resistance. In order to transfer a significant amount of the heat from the resistance element surface to the near wellbore reservoir region, a very high surface temperature must be generated. High surface temperatures cause thermal coking of petroleum product and degradation of insulating and other material with consequent failure of the device. As a result, this type of electrical heater is no longer commonly used in the petroleum industry.
Another type of electrical heating device that has been extensively tested in the field involved the isolation of one or more electrodes in the well production casing, or liner string, which are used to conduct electrical current via the connate water or conductive material in the reservoir. With this type of device, the electrical resistivity of the reservoir itself is utilized as a heating element. Again the heat generated within a specific location is proportional to the resistance and the square of the current passing through that region. Several configurations of equipment have been proposed and tested to effect near wellbore heating in this way. One uses production casing in the well with a coating of electrical insulation added to its surface except for the region where the current is to pass to the reservoir. Electrical current is passed to the reservoir by connecting one pole of an AC electrical power source to the production casing and the other pole to a ground electrode. These systems proved to be impractical because of difficulties in maintaining a perfectly impermeable electrically insulating membrane on a long string of production casing that must withstand rough handling in the field and extremes of temperature during installation. In addition, the insulation degrades quickly due to overheating causing the system to become inefficient and ineffective after an impractically short period of operation. This method also required completion of the subject well in a specific manner such that installation in an existing well is impractical in most instances.
Other system configurations based on the concept of passing electrical current into the reservoir via electrodes use two or more sections of electrically non-conducting materials inserted in the casing string to isolate the electrode(s). With these configurations, AC electrical power is conducted to the electrodes by a power cable or by the well's production tubing that has been suitably insulated for the purpose. While the published results of field tests of these electrode systems have shown considerable promise for effectively stimulating oil production, the systems have been prone to premature failure and have several major inherent disadvantageous characteristics which have limited their acceptance by the petroleum industry. One inherent problem with electrode systems is that they require either a new well with a completion designed especially for the system or a very extensive and often impractical re-working of an existing well. Another problem is that oil reservoirs are not homogeneous and are often formed of layers of sediment having differing physical characteristics. Layers of sediment with differing physical characteristics, respond differently to thermal conditioning. With present systems this inevitably leads to uneven heating, as they lack the ability to differentiate between layers. The least productive layers, which typically have low resistance, conduct most of the current such that the required voltage for a reasonable release of heat in such layers, is inadequate to effectively heat the production layers which are typically composed of high resistance material. A further limiting characteristic of the method is the highly non-linear voltage gradient existing at the interface between the electrode and isolation section. Most of the energy is released near the ends of the electrodes resulting in high temperatures in a local area with little increase in temperature over the bulk of the electrode. In order to release enough heat to stimulate productivity the electrode to isolator connection can reach uncontrollably high temperature levels causing failure of the electrode and/or adjacent insulating and completion materials. Electrode systems require the use of single phase alternating current with the return current external to the supply cable. Alternating current is used rather than direct current in order to maintain electrolytic corrosion in the well to an acceptable level. Electrode systems that utilize either a power cable or an insulated tubing string to deliver power to the electrodes can be operated at AC frequencies below normal power frequencies. This is done to minimize overheating that can occur in the power delivery system due to the induced currents that are generated in the ferromagnetic tubulars of the well and accessories. Despite operating at quite low frequencies, damaging overheating can result due to the high current required to deliver significant power with the low resistance common with this configuration. Electrode systems are fundamentally limited in the combined length of the electrodes being used, and, therefore, the thickness of exposed reservoir face that can be heated. The reason for this is that the efficiency of the electrode system is determined by the ratio of the electrical impedance of the electrode divided by the electrical impedance of the entire system. The impedance of the electrode is inversely proportional to its length and a function of the electrical resistivity of the reservoir formation in contact with the electrode. The resistivity of oil bearing formations varies greatly depending primarily on its porosity and its saturation with oil, water and gas. Also, the resistivity of the formation declines as its temperature increases, therefore, the impedance of the electrode and the efficiency of the system declines as the formation temperature increases. One particularly intractable problem with electrode systems is that electrical tracking seems to occur inevitably across the surface of insulators exposed to the produced fluids from the wells. These fluids are often composed of two liquid phases, oil and salt water. At and below the electrical potential differences used in these systems the movement of a stream of conductive salt water across the isolating section causes sparking which initiates a carbon track as the stream of conductive liquid breaks or makes contact with the metallic elements on either end of the insulator. With each spark additional conductive material is deposited that effectively extends the track thereby reducing the length of the isolating section until a flash over renders the system inoperative. A similar phenomenon may take place within the reservoir, thus adversely affecting the reservoir characteristics and causing unstable electrical operating conditions. If operations continue, production casing or isolator failure can occur, requiring abandonment or expensive recompletion of the well. Operation under these circumstances is characterized by sudden current surges which cause the failure of delivery fuses and or electrical cables. As a result of all these factors the system has a short operating life and limited application.
Horizontal wells, that is petroleum wells in which the production completion zone lies in a horizontal or near horizontal plane, generally use steam to increase productivity, with the same general limitations affecting vertical or near vertical wells. U.S. Pat. No. 5,539,853 which issued to Jamaluddin in 1996 discloses a system in which heating elements are deployed within a tubing section within the production zone with hot gasses passing over the elements and then discharging to the reservoir. Since the gases must be supplied from the surface and penetrate into the formation, a counterflow condition exists which is similar to that of steam injection. Since the ambient gravitational and reservoir pressure gradients are disrupted by the counter current flow of the steam or gas, the full effect of heat addition is compromised.